The prospect for potential clinical applications of electron paramagnetic resonance (EPR), i.e. the feasibility of using EPR on in vivo samples containing spin-labeled compounds, is contingent on whether sensitivity is sufficient and skin depth effects are negligible at radiation frequencies that are not, in a clinical context, hazardous. Since the radiation customarily used for EPR of in vitro samples lies in the microwave frequency range and may, therefore, be hazardous, in the proposed research sensitivities will be assayed at lower, radiofrequencies--60, 100 and 340 MHz--by means of conventional continuous wave techniques. Measurements will be made as a function of sample size, sample viscosity and with varying relative rf coil and sample geometry. In order to model the dielectric and conductivity properties of biological tissues for these tests, the spin-label sample will be placed in a much larger gelatin sample, with the position of the spin-label sample relative to a rf coil at the surface of the gelatin specified. By this means skin depth effects will be estimated, i.e. an approximate rf field distribution within the gelatin model due to a coil at its surface determined. Should the results from the proposed research be encouraging, then a second stage (not covered in this proposal) will be undertaken. In this second stage a comparison of sensitivity for pulsed versus continuous wave techniques will be made. Also, studies of radiation effects on small animal models will be carried out using pulsed radiation in which the duty cycle and frequency are varied. From the results of both stages it should be possible to determine a frequency for which sensitivity is maximized consistent with negligible health hazards. Given positive results from the first two stages, the transfer of EPR techniques from in vitro to in vivo cases may be undertaken for specific, clinical applications in which exogenous spin-labeled compounds are used to monitor body biochemistry--for example, drug transport to and consumption by targeted organs, pH in body fluids, O2 transport and consumption.